The present invention generally relates to medical devices. In particular, the present invention relates to a medical device for use in a body lumen that includes a binary alloy, such as nickel-titanium, that operates exclusively in the austenitic phase.
In the last two decades, binary nickel-titanium (NiTi), or nitinol, alloys have seen an increase in a variety of uses in medical devices. One benefit of applying nitinol to medical devices is that the alloy has tremendous elasticity and shape memory characteristics. Furthermore, as used in medical devices, this material is highly biocompatible, kink resistant, fatigue resistant, and has many other beneficial engineering attributes. One beneficial engineering attribute of nitinol is superelasticity, also commonly referred to as pseudoelasticity. Superelasticity or pseudoelasticity refers to this material's ability to undergo extremely large elastic deformation.
In one particular application, nitinol has found use in self-expanding stents. Historically, stents were not self-expanding but deployed by a balloon. Balloon expanded stents were and are used in conjunction with balloon angioplasty procedures with the intent to reduce the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support. Examples of intravascular stents can be found in, for example, U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 5,092,877 (Pinchuk); and U.S. Pat. No. 5,514,154 (Lau et al.).
For balloon expandable stents, the stent is positioned over the balloon portion of the catheter and is expanded from a reduced delivery diameter to an enlarged deployment diameter greater than or equal to the inner diameter of the arterial wall by inflating the balloon. Stents of this type are expanded to an enlarged diameter through deformation of the stent, which then engages the vessel wall. Eventual endothelial growth of the vessel wall covers the stent.
Nitinol then found use in self-expanding stents, where deployment was a result of either shape-memory effect or superelasticity in the material rather than by an inflating balloon. The stent once released from its delivery system assumed a pre-set shape in the body lumen.
Self-expanding stents are used to scaffold the inside circumference of a tubular passage such as an esophagus, bile duct, or blood vessel. Likely the more popular area of application is the cardiovascular system, where a self-expanding stent is used subsequent to balloon angioplasty. Cardiovascular stents currently available in the United States are made of stainless steel, and are expanded against the vessel wall by plastic deformation caused by the inflation of a balloon placed inside the stent. Nitinol stents, by comparison, are self-expanding. Instead of being deformed to the vessel diameter by a balloon catheter, the nitinol stent returns to its non-deformed, equilibrium shape. Examples of stents made of a superelastic nitinol alloy are disclosed in U.S. Pat. No. 4,503,569 (Dotter); and U.S. Pat. No. 4,665,906 (Jervis).
The benefits of using a superelastic nitinol material for self-expanding stents are primarily related to its large recoverable strain. The biocompatability of nickel-titanium is also an attractive benefit for use of this material in stenting applications, because the stent remains in the patient as part of the treatment.
The use of nickel-titanium as a balloon-expandable stent is less common. At present, the PARAGON Stent is a balloon-expandable nickel-titanium stent. The balloon-expandable and scaffolding capabilities of the PARAGON Stent are accomplished by setting the austenite finish temperature (Af) at about 55 degrees C. or well above body temperature. The stent is therefore completely martensitic before, during, and after balloon deployment. A significant disadvantage of such a balloon-expandable nitinol stent in its martensitic phase is that martensite is very soft. Therefore, the scaffolding function and hoop strength of the stent are diminished.
As briefly described above, superelasticity or pseudoelasticity, refers to the highly exaggerated elasticity or spring-back observed in many nickel-titanium alloys deformed above its austenite start temperature (As) and below the martensite deformation temperature (Md). Hence, nickel-titanium alloys can deliver over fifteen times the elastic motion of a spring steel. The martensite deformation temperature (Md) is defined as the temperature above which martensite cannot be stress-induced. Consequently, nickel-titanium remains in its austenitic phase throughout an entire tensile test above Md.
The evolution of superelastic and shape memory alloy stents has progressed to the use of ternary elements in combination with nickel-titanium alloys to obtain specific material properties. Use of a ternary element in a superelastic stent is shown in, for example, U.S. Pat. No. 5,907,893 (Zadno-Azizi et al.). As a general proposition, there have been attempts at adding a ternary element to nickel-titanium alloys as disclosed in, for instance, U.S. Pat. No. 5,885,381 (Mitose et al.).
Nitinol alloys contain more nickel than does 316L-grade stainless steel, the most common material used for medical devices. It is recognized that nickel is considered toxic. As nitinol oxidizes, it forms a titanium oxide layer (TiO2), with small islands of pure nickel on the surface, or, depending on the treatment, with no nickel present at the surface. Accordingly, nitinol is highly biocompatible and more so than stainless steel.
Nitinol has found its way into other medical device applications. An example of a guide wire made of superelastic nitinol for performing angioplasty or vascular intervention procedures is disclosed in U.S. Pat. No. 6,068,623 (Zadno-Azizi et al.).
Still other medical device applications for nitinol include filters. Pulmonary embolism is the sudden obstruction of a blood vessel by blood emboli, the emboli typically formed in the veins of the pelvis and lower extremities of a person's body. Because migration of the blood emboli to the pulmonary artery can interrupt the oxygenization process of the lungs, the disease has a high mortality rate. Vena cava filters have been developed as one method for preventing pulmonary embolism. Such a device is disclosed in U.S. Pat. No. 5,350,398 (Pavcnik et al.) Nitinol has been used in fabricating vena cava filters. One discussion of such a use can be found in “A Vena Cava Filter Using Shape Memory Alloy,” M. Simon, R. Kaplow, E. Salzman, D. Freiman, Radiology, Vol. 125, pp. 89–94, October 1997. According to the authors, their vena cava filter uses shape memory to deploy into its pre-set, austenitic shape, where the reversion to the pre-set, austenitic shape is triggered by application of heat.
In view of the foregoing, there is still a need for a medical device that operates exclusively in the austenitic range. Such a device is further compatible with balloon catheters. If fabricated into a stent, such a device would have great radial hoop strength. With all of these benefits, the austenitic medical device would also be highly biocompatible, have greater MRI compatibility, and would be more flexible than medical grade stainless steel.